Methods, apparatuses, and systems for micromanipulation with adhesive fibrillar structures

ABSTRACT

The present invention are methods for fabrication of micro- and/or nano-scale adhesive fibers and their use for movement and manipulation of objects.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a US national phase application of PatentCooperation Treaty international application PCT/US2012/071168, titledMETHODS, APPARATUSES, AND SYSTEMS FOR MICROMANIPULATION WITH ADHESIVEFIBRILLAR STRUCTURES, filed Dec. 21, 2012, which claims priority of U.S.Provisional Application 61/630,954, titled METHODS, APPARATUSES, ANDSYSTEMS FOR MICROMANIPULATION WITH ADHESIVE FIBRILLAR STRUCTURES, filedDec. 22, 2011, both are incorporated by reference herein.

STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with partial government support under NationalScience Foundation grant CMMI-0800408. The government has certain rightsin this invention.

FIELD OF THE INVENTION

This invention relates to devices and systems for movement andmanipulation of macro- and micro-scale objects. In particular, thepresent invention relates the fabrication and use of micro- and/ornano-scale fibers as adhesive structures that can controllably adhereto, move, and release from macro- and micro-scale objects.

BACKGROUND OF THE INVENTION

Geckos are one of nature's most agile and power-efficient climbers dueto their strong, highly repeatable, high speed, and controllableattachment and detachment capabilities on a wide range of smooth andslightly rough surfaces. Such capabilities are a result of angled andhierarchical micro and/or nanoscale fibrillar structures on their feet,which have saucer shaped tip endings. These micro/nanostructures canexhibit repeatable adhesive strengths up to 200 kPa on smooth and rigidsurfaces such as glass. The attachment strength of gecko foot-hairs hasbeen shown to be rooted in intermolecular forces such as van der Waalsforces, which exist between all surfaces and is fairly insensitive tosurface chemistry. This generic attachment principle enables the animalto climb on a wide range of surface materials. The importance ofgeometry, size, material type, and surface physics of these biologicalfoot-hairs to their adhesion strength, rather than their surfacechemistry, leads these biological adhesives to be called structuredadhesives. Many researchers have proposed methods to design andfabricate such synthetic micro/nanostructured adhesives inspired bygecko foot-hairs.

In addition to high attachment strength, biological micro/nanofibrillarstructures exhibit highly controllable adhesion. The controlled adhesionand shear strength of gecko's angled fibrillar structures is dependenton mechanical deformations induced by vertical and lateral loading ofits feet, which can actively control the contact area between thestructures and the substrate. It has been demonstrated that geckofoot-hairs have a friction ratio of around 5 to 1 comparing the with toagainst hair tilt directions.

Synthetic structured adhesives have been designed in an attempt to mimicthe strength and controllability of these biological foot-hairs. It hasbeen shown that when stiff polymer microfiber arrays are angled theyexhibit anisotropic behavior of shear strength with a ratio of 45 to 1between dragging resistance in with and against fiber tilt directions.However, in both of these vertical and angled cases, the microfibers hadlow adhesive strength. Researchers have used multi-wall carbon nanotubes(MWCNT) to create a structured surface with even smaller features thatexhibit adhesive strength of 100 kPa and shear strength of 80 kPa.Similarly, embedding MWCNT arrays in polymer backing showed enhancedfriction, but these MWCNT surfaces lacked controllable adhesion.

In the study with results closest to the strength and controllability ofbiological foot-hairs, researchers have developed elastomer, angledpolymer fibers with angled mushroom shaped tip endings whichdemonstrated interfacial shear pressures of 100 kPa and adhesionpressure of 50 kPa. These structures exhibited controlled shear andadhesion strength: with-to-against friction ratios of around 5 to 1 andadhesion ratios of 35 to 1. Subsequently, surface treatments have beenused to enhance adhesion of polymer microfibers in air and under water.In a different approach to adhesion control, thermal control has beenused on shape memory polymer fiber arrays.

The aforementioned preload-and shear-controlled adhesion and frictionproperties could be one of the major reasons why biological geckofoot-hairs can shed dirt particles in dry conditions. Researchers havedemonstrated that dirt microparticles much larger than the fiber tipdiameter could be shed from the gecko's foot after it is attached to anddetached from a clean glass substrate in many cycles, a process termedcontact self-cleaning. Such contact self-cleaning property has been alsobeen shown in synthetic polymer fiber adhesives by shear loading. Thesestudies suggest that micro/nanostructures could also be used forpick-and-place manipulation of micro or macroscale parts since theyenable controlled attachment (pick) and detachment (release). Therefore,microstructured adhesives inspired by these biological structures haverecently been used for manipulation at the micro and macroscale.

Researchers have presented elastomer micropyramidal structures asadhesion controlled micromanipulators. These microstructures usedvertical compression induced contact area control such that there was arelatively large contact area when sufficiently large compressive loadsbuckled the microstructures. If pulled away quickly, the planar part waspicked up with a high pull-off force because rate-dependent effectsenhanced the adhesion strength further. After the part was picked, thebuckled elastic structures reverted to their original shapes. This shaperecovery significantly reduced the contact area, and thus, adhesion,between the pyramid structures and the part and enabled easy partrelease. The maximum ratio of pick to release adhesive forces was 1000to 1. But, this manipulator had small holding forces after lifting thepart from the donor substrate. Though the holding force was not measureddirectly, the observed contact area while holding a part was threeorders of magnitude less than while picking up a part, indicating aholding force less than 1 nanonewtons, which could be a problem forheavy parts or for mechanical disturbances during transfer of the parts.Researchers have addressed this limitation by removing themicropyramidal structures of the manipulator and used shear displacementcontrol to reduce attachment strength, at the cost of a reduced pick torelease force ratio, but the force ratio was not presented. Researchershas utilized angled nanofibers with high shear strength to transfer thinfilm transistor (TFT) displays under vacuum or air as a macroscalemanipulation demonstration. However, micron scale part manipulation wasnot demonstrated and the nanofiber array required a constant applicationof shear force for strong adhesion.

SUMMARY OF THE INVENTION

The present invention is a gecko foot-hair inspired angled pillarmicrostructure with flat or round tip ending shape (FIGS. 1A-C) toimprove the versatility and simplicity of the elastomer micro and/ornanostructure-based pick-and-place manipulation of macro and microscaleparts. Scanning electron micrographs taken from an isometric viewpointof FIG. 1A is the round tip micropillar, FIG. 1B is the flat tipmicropillar, and FIG. 1C is a side view of the flat tip pillar attachedto a silicon platelet part. The micropillars are made of elastomericpolyurethane. The white scale bars represent 50 μm of length. Thepresent invention is described in two simple control methods forreducing attachment strength of the pillar during the part release:vertical displacement control and shear displacement control. Picking ofthe part is accomplished in the same way for both control methods, byvertical compression of the tip to the part and rapid retraction tomaximize adhesion strength. Tip adhesion with the part can be due to anyattractive forces such as van der Waals, hydrogen bonding, capillary,magnetic, vacuum suction, mechanical interlocking, or electrostaticforces. During transfer of the part, the attachment between the pillarand the part is secure enough to withstand sudden impacts anddisturbances as well as the weight of the part, issues that could be alimiting factor with some of the prior art. During release of the partonto a receiver substrate, the contact area of the pillar to the part isdrastically reduced by the deformation of the pillar due to either thevertical or shear displacement control method. In contrast to the priorart nanofiber arrays, the parts can be picked and released in bothadhesion and shear modes in the approach described here.

Such compliant micromanipulators are simple and inexpensive tomanufacture, easy to integrate into optical microscopy infrastructure,and can operate in air, in vacuum and under liquid. Finally, suchcompliant polymer micropillars are safe for use with fragile parts, and,due to exploiting intermolecular forces, are effective on mostmaterials. This micromanipulation system's ease and effectiveness willbe a benefit to the assembly and packaging of microelectromechanicalsystems and optoelectronic and flexible electronic devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-C show scanning electron micrographs of adhesive fibersfabricated and used according to the present invention;

FIGS. 2A-D show schematic representations of the processes forfabrication of the adhesive fibers according to the present inventionfor flat tips;

FIGS. 2E-I show schematic representations of the processes forfabrication of the adhesive fibers according to the present inventionfor rounded tips;

FIGS. 3A-F show video stills and schematic drawings of the contactbetween an adhesive fiber fabricated according to the present invention,and an object to be manipulated in accordance with the presentinvention;

FIG. 4 shows force-distance curves for a round tip adhesive fiberfabricated according to the present invention;

FIG. 5 shows force-distance curves for a flat tip adhesive fiberfabricated according to the present invention;

FIG. 6 shows adhesive forces of flat and round tip adhesive fibersmeasured during pull-off from a glass substrate;

FIG. 7 shows adhesive forces of flat tip adhesive fibers measured atpull-off for different shear displacements and retraction speeds;

FIGS. 8A-F show video snapshots of the steps of a pick-and-placemanipulation of an object according to the present invention;

FIGS. 9A-D show images of the use of an array of adhesive fibers tomanipulate and object;

FIG. 10 illustrating the radius of the round tip fiber; and

FIG. 11 illustrates examples of fiber tip forming operations.

DETAILED DESCRIPTION OF THE INVENTION

This invention describes method for fabrication of micro- and/ornano-scale adhesive fibers and their use for movement and manipulationof objects.

Fabrication

Previous work has shown the importance of the tip geometry on theadhesion and friction of microfiber adhesives. In the present invention,two distinct pillar types are described that are similar in allgeometric and material parameters except for the shape of the tip. Thefirst type has a flat tip, the surface of which is parallel to the planeof the backing layer. The second type's tip is in the shape of a roundedbump with a given curvature. The principle of the fabricationmethodology of the present invention is based on an optical lithographybased microstructure fabrication followed by a molding basedreplication. Angled elastomer micropillars can be fabricated byreplicating positive pillars fabricated by directional reactive ionetching or SU-8 lithography. The latter method is described in detaildue to its simplicity.

Flat tips 8 can be formed from negative molds 10 made from materialssuch as SU-8 because the polymer tips 8 (FIG. 2B) cure against theatomically smooth outer surface 9 of a silicon substrate 12 (FIG. 2A).The angled flat tip micro- and/or nanoscale pillar 22 fabricationprocess starts with the patterning of, for example, an SU-8 mold 10. Asilicon substrate 12 is spin coated with, for example, a 160 nm thickanti-reflection layer 14 (for example, XHRiC-16, Brewer Science). On topof the anti-reflection layer 14, SU-8 negative photoresist materiallayer 16 (for example, SU-8 50, Microchem) is spin coated andsoft-baked, in one embodiment into a 90 μm thick layer 11. Thephotoresist material layer 16 can have a thickness 11 ranging from about10 nm to about 10 mm. To fabricate the angled pattern 18, the siliconsubstrate 12 with soft-baked SU-8 photoresist material layer 16 ismounted on an angled stage (not shown) and exposed to ultraviolet (UV)light, followed by a post-exposure baking and development. The resultingSU-8 mold 10 has a negative pattern, i.e. composed of angled holes 20(FIG. 2A), which are hard baked, in one embodiment at 180° C. for 3 minto induce further crosslinking. An outer surface 9 of the siliconsubstrate 12 is exposed between adjacent angled holes 20 of theplurality of angled holes. The SU-8 angled micro- and/or nanoscalepillars 22 are then molded with a liquid polymer 17 (FIG. 2B), whichcreates arrays 13 of elastomer micro- and/or nanoscale pillars 22 (FIG.2C). To facilitate the delamination of the micro- and/or nanoscalepillars 22 (for example, polydimethylsiloxane (PDMS)) from the SU-8 mold10 (FIG. 2C), the mold 10 can be exposed to the vapor of a desiccator,such as tridecafluoro-1,1,2,2-tetrahydrooctyl-1trichlorosilane for 60min. Array 13 is isolated in FIG. 2D to illustrate a single micro-and/or nanoscale pillar 22A. Liquid polymer 17 can be a thermosetelastomer such as silicone, rubber, polydimethylsiloxane, acrylate, andpolyurethane, or any suitable thermoplastic elastomer. FIG. 10illustrates space S between adjacent pillars or fibers 22 and length Lof each pillar or fiber 22. Spacing S can range from about 1 nm to about500 mm. Length L can range from about 1 nm to about 100 mm

Rounded tips 24 are formed from positive SU-8 photoresist molds 26because the tips 28 of the standing SU-8 photoresist posts 30 are etchedmore along the perimeter 29 creating a curving of the top surface 32 oftips 28 to form a radius R (FIG. 10) when etching SU 80 photoresistposts 30 to form an angled round tip micro- and/or nanoscale pillar 34,where radius R is in proximity of the center line C/L of pillar or fiber22. Radius R can range from zero to infinity. By tuning the exposure anddevelopment times by a trial and error process, the curvature of the tip28 can be controlled. The angled round tip micro- and/or nanoscalepillar 34 described herein is fabricated using the lithography (forexample, using SU-8) and molding techniques described in previous works.In a similar approach to the flat tip pillar fabrication, in oneembodiment illustrated in FIG. 2E, SU-8 photoresist (SU-8 2050,Microchem Corp.) is spun on a fused silica substrate 36 and exposedthrough a mask (not shown) by angled UV light (MA-56, Karl Suss). Thedifference in round-tip fabrication from flat-tip fabrication is in themold: the round-tip mold is a positive pattern, i.e. composed of angledmicro- and/or nanoscale pillars 34 (FIG. 2E). The SU-8 angled micro-and/or nanoscale pillars 34 are then molded with a silicone rubber 38(FIG. 2F) (for example, HS II RTV, Dow Corning), which serves as thenegative pattern mold 40 (FIG. 2G) for creating arrays 43 of elastomermicro- and/or nanoscale pillars 42 (FIG. 2H). One embodiment of thecurvature of the round tip structure 28 can be characterized withinterferometric profilometry and the radius of curvature can be 380 μm.The round tip radius R can range from about 1 nm to about 100 mm. Thecurvature of the tip 28 is a controllable design parameter, which canrange from 10 ths of microns radius of curvature up to infinite radiusof curvature (i.e. flat tip). Array 43 is isolated in FIG. 2Iillustrating a single micro- and/or nanoscale pillar 42A.

Photolithography of SU-8 using a UV light source is a relativelyaccessible and established process, but it is not the only approach forproducing angled polymer micro and/or nanostructures 13, 43. Researchershave adapted the process of deep reactive ion etching (DRIE) to theangled etching of polysilicon. This allows for a higher degree ofcontrol and repeatability in the structures' geometry, but requires aless common fabrication technology. Conversely, issues of consistency inSU-8 fabrication can be addressed by identifying and isolating singlestructures with desired geometries.

The material used as the final micro- and/or nanoscale pillar arrays 43can be a variety of polymers 44 (FIG. 2G). In one embodiment, the arrays43 are formed with ST-1087 (BJB Enterprises, Inc.), a polyurethaneelastomer with a Young's modulus of 9.8 MPa and an effective work ofadhesion on glass of 32 mJ m⁻². Young's modulus can range from about0.01 MPa to about 100,000 MPa. Effective work of adhesion can range fromabout 10 mJ m⁻² to about 2,000 mJ m⁻². This particular polyurethane hashigh tensile strength and high surface energy while remaining opticallytransparent.

The geometry of the arrays 43 can be characterized with opticalmicroscopy (TE200 Eclipse, Nikon), interforemetric profilometry(NewView™ 7300, Zygo), and scanning electron microscopy, (SEM, Hitachi2460N). All arrays 43 were molded onto substrate 46, for example ˜2 mmon a side, to provide a rigid, transparent backing and to ease manualhandling (FIG. 2H). The molding process resulted in the substrate 46being covered in several hundred micro- and/or nanoscale pillars 42,with a polyurethane backing layer 48, for example, less than 20 μm thickbetween micro- and/or nanoscale pillars 42 and the rigid substrate 46.This thin backing layer 48 is advantageous because it reduces anycomplicating effects of the soft backing. Polyurethane backing layer 48can have a thickness ranging from about 1 nm to about 100 mm. Substrate46 can be made of glass, acrylic or other suitable material.

In summary, FIGS. 2A-I are schematic representations of the twofabrication processes. For flat tips 8, FIG. 2A starts with an SU-8photoresist negative mold 10 on a silicon substrate 12; FIG. 2Billustrates pouring the liquid polymer 17 into the SU-8 photoresistnegative mold 10 and curing the liquid polymer 17 directly in the SU-8photoresist negative mold 10; FIG. 2C illustrates the positive micro-and/or nanoscale pillar array 13 of micro- and/or nanoscale pillar 22that is the product of the above mentioned molding process on a glasssubstrate 19; and, FIG. 2D illustrates the final selection and isolationof a single micro- and/or nanoscale pillars 22A. For rounded tips 28,FIG. 2E starts with an array of positive SU-8 micro- and/or nanoscalepillars are etched more along the perimeter 29 creating a curving of thetop surface 32; FIG. 2F illustrates pouring a liquid Silicone Rubber 38into the SU-8 photoresist positive mold 26 and curing a negativeSilicone rubber mold 40; FIG. 2G illustrates a liquid polymer 44 beingpoured into the negative Silicone rubber mold 40 where it is cured; FIG.2H illustrates the positive array 43 that is the product of the abovementioned molding process on a glass substrate 46; and FIG. 2Iillustrates the final selection and isolation of a single micro- and/ornanoscale pillar 42A. Fabricating structures from a negative photoresistmold 10 (FIGS. 2A-D) results in angled micro- and/or nanoscale pillars22A with flat tips 8 (see FIG. 1B). Replicating positive photoresistmolds 26 (FIGS. 2E-I) results in angled micro- and/or nanoscale pillars42A with round tips 28 (see FIG. 1A).

Below is a table of the fiber parameters used for the experimentsdescribed above:

Fiber Type Round Flat Length (L) 75 μm 95 μm Height (H) 72 μm 89 μmAngle of inclination (a) 16° 20° Diameter (D) Elliptical 35 μm (35 μm,45 μm) Radius of Curvature of 380 μm N/A tip (R) Young's Modulus 9.8 MPa9.8 MPa Work of adhesion 32 mJ m⁻² 32 mJ m⁻²

Object Manipulation

Experimental Setup

In order to characterize the performance of the microstructures inobject manipulation tasks, a custom experimental system was employed.This system is based upon automated flat-punch indentation setupspreviously used in adhesion characterization experiments. Using aninverted optical microscope (TE200 Eclipse, Nikon) as the base for thefixturing as well as the source of visual feedback, a vertical axis ofmotion and sensing was mounted such that the point of intersectionbetween the pillar micromanipulator and substrate would occur at thefocal range of the optics. The vertical axis motion was provided by alinear motorized stage (MFA-CC, Newport) with submicron positionalaccuracy and a speed range from 1 μm s⁻¹ to 2500 μm s⁻¹. The verticalstage was mounted to a two axis manual linear stage (462 Series,Newport) and a two axis goniometer (GON40-U, Newport) to align theadhesive sample with the optics and the substrate.

Sensing was achieved through a high resolution load cell (GSO-10 andGSO-30, Transducer Techniques), which was used with a signal conditioner(TMO-2, Transducer Techniques). The video was captured through a colordigital camera (DFW-X710, Sony) connected to a desktop computer (AspireASE380-ED500U, Acer) operating Linux (Ubuntu 7.10 Gutsy Gibbon). Theforce data was captured as an analog voltage signal through a dataacquisition board (NI PCI-6259, National Instruments) mounted in thecomputer, and all motion control was achieved through commands sent fromthe computer to a motor controller (ESP300, Newport) to which themotorized stage was connected. All data capture and motion control wasmanaged by custom software running on the computer.

The experimental control parameters included the speed of approach ofthe adhesive sample to the substrate, the initial amount of compressiveload applied (preload), the amount of displacement in the compressivedirection after preloading, the amount of displacement in the lateralshear direction after preloading and finally the pull-off speed. Thevariable which was measured was the applied normal force on themicropillar during loading and retraction. Visual feedback from thevideo recording gave qualitative information regarding the mechanics ofthe structures. Contact area visualization and fiber tip geometry wasenhanced by interference patterns in 546 nm wavelength monochromaticgreen light with the corresponding optical filter during the pick andrelease of parts.

It is important to note that the control variable in all experiments wasdisplacement, either vertical or shear. Force based control failed tocapture intermediate load states because of the unstable nonlinearresponse of the pillars under compression. from about 1 nm up to about100 mm of compressive displacement

Vertical Displacement Experiments

A typical adhesion characterization experiment would have the structuraladhesive sample mounted on the vertical axis such that the adhesive waspointing downward towards the substrate mounted to the microscopefixture. After approaching at 1 μm s⁻¹ (constant for all tests) andachieving a desired preload of 0.05 mN (constant for all tests) thevertical stage would continue to compress the pillar for a prescribeddisplacement. Once the prescribed compressive displacement was achieved,the vertical linear stage retracted the micropillar at a constantvelocity. The maximum tensile force during pull-off was recorded as theadhesive force.

Shear Displacement Experiments

In the case of applying shear displacement during the part release, themanual linear stage was employed after the compression step wascompleted, but before retraction. After achieving the prescribedcompressive displacement the motorized linear stage paused for 10seconds to allow the experimenter to displace the pillar laterallythrough the use of the manual linear stage. As before, the maximumtensile force was recorded as the adhesive force.

Demonstration of Pick-and-Place Manipulation

A micromanipulator composed of a single angled pillar was used for allempirical characterization as well as demonstrations of pick-and-placeof 100×100×3 μm³ silicon platelets. The silicon parts were fabricatedaccording to well-known techniques. The manipulation of thecentimeter-scale glass slide was conducted with an array of 100 roundtip pillars arranged in a square packed pattern with 120 μm center tocenter distance.

Results

Effect of Tip Shape

The two micropillar geometries of the present invention wereinvestigated, one with a flat tip and one with a rounded tip. The flattip pillar's contact process is captured in micrographs and sketches inFIGS. 3A-F, where the contact area micrographs show that the “toe”(defined as the edge of the tip further away from the base of thepillar) peels up after a critical amount of compressive displacement(FIGS. 3C-F). A rounded tip pillar's contact process resembles the flattip process, except for the lack of a critical peeling event, rather,the tip slides along the surface until the entire pillar is bent overand prone. Each column shows three corresponding images: (top row) videostills of the flat tip pillar's contact to a smooth, flat glass as seenthrough an inverted microscope with monochromatic green lighting;(middle row) side view video stills of the profile of the flat tippillar as it is vertically compressed; (bottom row) side view schematicsof the pillar profile during vertical compression and retractionincluded in order to aid in visualizing the process. The process beginswhen the tip barely makes contact (FIG. 3A) before fully contacting thesurface; FIG. 3B illustrates additional compression causes peeling dueto mechanical instability; FIG. 3C is an illustration after which thetip continues to slide along and peel away from the surface; FIGS. 3Dand 3E illustrate, upon retracting, the contact patch is seen to beminimized;

FIG. 3F illustrates the scale bars for each row is included in FIG. 3A,and all represent the same length: the diameter of the flat tip, 35 μm.The flat tip diameter can range from 1 nm to 100 mm.

The behavior of these contact processes was captured quantitatively inforce versus displacement graphs, FIG. 4 and FIG. 5 for round and flattips, respectively. The graphs show that there is hysteresis in theloading and unloading of the micropillar which influences the pull-offforce: by compressively loading the pillar, either rounded tip or flattip; it first makes good contact resulting in high pull-off forces.Further compression causes the tip surface to either peel away in thecase of the flat tip, as indicated by the sharp drop in the measuredcompressive force seen at point (c) of FIG. 5. The cause for thismechanical instability relates to the nonlinear stress distribution atthe tip-substrate contact face.

Now turning to FIG. 4, Force-distance (FD) curves of the round tippillar obtained from indenting it onto a glass slide. FD data of theloading, at a constant compression rate of 1 μm s−1, can be seen as theoverlapped red lines flowing from left to right. The pillar wasretracted at 100 μm s−1 after different distances of verticalcompression were obtained, which created the separate blue lines flowingfrom top right to bottom left at different intervals. The schematics ofthe side view of the micropillar profile are based on optical microscopyobservations captured via video and correlated to the empirical FD data;the schematics show the physical behavior at points of interest alongthe curve, highlighted by call-out boxes. Following the red FD curvefrom the origin (at the intersection of the dashed lines) to the pointof vertical compression at (a) then retracting along the blue curveshows how to obtain a high adhesive force, i.e. maximum tensile force,at point (e). The adhesive force is significantly reduced if beginningfrom the origin again, compress the pillar until it is prone, as inpoint (c), before retracting to point (d), where it is seen that onlyedge contact is made at the moment of separation. In the case of theround tip pillar, the tip slowly slides until the pillar is bent andmaking contact on its side (point (c)). By controlling the vertical orshear displacement, the contact area of the pillar is controlled, andthereby control whether it is in the pick state, defined as when thepillar exerts the maximum pull-off force, or in the release state,defined as when the pull-off force is minimized. However, the pickingretraction speed can range from about 1 nanometer/second up to about 1meter/second.

Now turning to FIG. 5, the flat tip pillar was compressed onto a glassslide at 1 μm s−1 then retracted at 30 μm s−1 to create force-distance(FD) curves. Loading is graphed as overlapping red lines flowing fromleft to right, and retracting data is graphed as intermittently spacedblue lines flowing from the top-right to the bottom-left. The schematicsof the micropillar profile are labeled to correspond directly with theinformation in FIGS. 3A-F, and the schematics are mapped by call-outboxes to the points along the FD curve where the mircopillar takes therepresented shape. Compressing the pillar from the origin (theintersection of dashed lines) to gentle contact at point (a), then topoint (b) before retracting to point (b)* gives a high adhesive force(i.e. maximum tensile force). Note that the shape of the pillar at (b)*is visually identical to its shape at (b), but it is in tension, sothe * is used to denote the difference. Compressing the pillar pastpoint (b) reveals a mechanical instability from point (c) to point (d)where the tip peels away suddenly, and by compressing even further, onlythe edge remains in contact at point (e) before retracting to point (f)where the pillar is making minimal contact at the moment of separation.However, the picking retraction speed can range from about 1nanometer/second up to about 1 meter/second.

Comparing the behavior of the flat tip pillar and round tip pillar undercompression shows that the flat tip has a larger pull-off force and asharper switch between the “pick” and “release” states, which is definedas the states where maximum and minimum pull-off forces are exerted,respectively (FIG. 6). The round tip has a less sharp distinctionbetween pick and release states, and a lower peak pull-off force. Thepick-to-release adhesive force ratio of the flat tip was found to be 35to 1 and the round tip had an pick-to-release adhesive force ratio of 26to 1. The adhesive forces of flat and round tip pillars measured duringpull-off from a glass substrate, after a given vertical displacement inthe compressive direction, are plotted for different retraction speeds.The slowest available retraction speed of the actuator, 1 μm s−1,minimized the adhesive forces for both the flat tip pillar (solid redlines connecting filled circles) and the round tip pillar (dashed redlines connecting open circles). The optimal pull-off speed for the flattip was 30 μm s−1 (solid blue lines connecting filled diamonds) and forthe round tip the optimal pull-off speed was 100 μm s−1 (dashed bluelines connecting open diamonds). Each data point represents the medianand the error bars indicate the minimum and maximum force values ofthree experiments. These results demonstrated how the flat tip pillarhas higher ratio of 35 to 1 and sharper switch between pick and releasestates than the round tip pillar with a ratio of 26 to 1 and a smoothswitch between states. The pick-to-release adhesive force ratio of theflat tip and round tip can range from about 1 to 1 up to about 100,000to 1.

It should be noted that the peak pull-off force of the flat tip wastwice that of the round tip, but the pick-to-release adhesive forceratio of the flat tip was less than twice that of the round tippick-to-release adhesive force ratio because the release-state of theround tip proved to exert a smaller force. It was observed that therelease-state depended on the roughness produced through fabricationstochasticity along the edge of the tip. In some embodiments of thepresent invention, bumps or other structures can be added along the edgewill reduce the release-state adhesion and enhance the pick-to-releaseratio. It was observed that alignment was a factor for improvingperformance of the flat tip, but could be neglected for the round tip.This difference may be used in some embodiments for applicationsrequiring easy or robust alignment. Due to the higher pull-off force andsharper distinction between pick and release states, the flat tip pillarcan be a preferred embodiment described below.

Effect of Shear Displacement

A previous investigation into flat tipped angled micropillars proposedan analytical model of the stress on the tip of the pillar. That modelsuggests that the angle of inclination of the pillar facilitates anuneven stress distribution during loading, causing the pillar to losetip contact. The present invention has shown how adhesive forces canmaximize or minimize simply by loading the pillars compressively (FIG.6). However, a similar control strategy can be implemented in someembodiments of the present invention by the addition of sheardisplacement.

Now turning to FIG. 7, the adhesive forces of flat tip pillars measuredat pull-off for different shear displacements and retraction speeds.Flat tip pillars where first contacted to glass with 4 μm of compressionto ensure maximum tip contact, then sheared laterally before beingretracted vertically at 1 μm s−1 (plotted with red circles), 10 μm s−1(green squares), or 30 μm s−1 (blue diamonds). Each data point and errorbars represent the median and minimum and maximum force values,respectively, of three tests. The pick-to-release adhesive force ratiowas found to be 39 to 1, but can range from about 1 to 1 to about100,000 to 1. In FIG. 7, it is observed that with no shearing and goodtip contact, achieved through 4 μm compression, there is a maximumpull-off force. Any amount of lateral shear displacement reduces thepull-off force until the release state is achieved for sheardisplacements of ≧8 μm. In this case, the repeatably observedpick-to-release adhesive force ratio of 39 to 1 is comparable to, butgreater than, compression only switching. From micromanipulation trials,it is found that using shear displacement control of adhesion to berepeatable and reliable. Shear displacement control was found to be morereliable than compression-only control during pick-and-placeexperiments.

Demonstration of Manipulation

Using the vertical or shear displacement based contact area control ofthe micropillars, the present invention enables pick-and-placemanipulation of objects. Such adhesion control can be seen in anassembly task in FIGS. 8A-F, where the indentation of the pillar intothe silicon microplatelet is critical for pick-and-place manipulation.Video snapshots from an inverted microscope show the steps ofpick-and-place manipulation of the silicon microplatelets: FIG. 8Aillustrates the micromanipulator contacting the first part, FIG. 8Billustrates the first part being picked up from the substrate, and FIG.8C illustrates bringing the first part in contact with the second part.With a loading condition of 4 μm of compressive displacement, the flattip pillar tip made good contact with the part and could lift it off ofthe glass slide as demonstrated in FIG. 8C. The tips of the micro-and/or nano-scale fibers can be compressed by from about 1 nm up toabout 100 mm of compressive displacement when making contact with parts.After moving to the desired location above the first part, the secondpart was released by increasing the downward (compressive) displacement(FIG. 8D) until the flat tip pillar lost tip contact (FIG. 8E). FIG. 8Eillustrates the pillar being slowly retracted. When the pillar wasretracted after tip contact was lost, the adhesion was low enough torelease the second part on top of first (FIG. 8F) thus completing themicroassembly. Alternatively, shear displacement can be used in additionto compression of the fibers or in place of compression of the fibers.When shear displacement is used along with compression of the fiber,shear displacement can occur before or after compression of the fibers.

The same principle used to control a single angled micropillar'sadhesive state can be applied to arrays of angled micropillars. A 4×1cm² glass cover slip was picked up and placed down with a 10×10 array ofround tip micropillars demonstrating the extensibility of this approachto larger length scales and heavier parts. FIGS. 9A-D are video stillsfrom a demonstration of the macroscale manipulation capability of a10×10 array of round tip micropillars. The cover slip has been outlinedand schematics representing the deformed state of any given pillar havebeen included to guide the reader: FIG. 9A is the array verticallydisplaced to sufficiently compress it to form a large contact beforeretracting rapidly and picking up the glass cover slip illustrated inFIG. 9B. FIG. 9C illustrates vertical displacement control is utilizedto compress the array of round tip pillars and induce edge contact whenbrought into contact again. FIG. 9D illustrates that retracting thearray slowly will enable the cover slip to be released.

Other embodiments can include picking and releasing two or more parts.These embodiments will require only a portion of the available pluralityof micro- and/or nano-scale fibers to contact to a part to be picked andreleased. Below is an example of the operational procedure for pickingand releasing two or more parts. For illustration purposes, two partswill be picked and released, but this illustration is not meant to limitthe invention to only simultaneously picking and releasing two parts:

Step 1: Providing a manipulation device with a plurality of micro-and/or nano-scale fibers, wherein each micro- and/or nano-scale fiberhas a tip;

Step 2: Contacting the first part with one or more tips of the pluralityof micro- and/or nano-scale fibers to form a first set of contactedtips;

Step 3: Picking up the first part and maneuvering it in position of thesecond part;

Step 4: Contacting the second part with one or more non-contacted (orfree or available) tips of the plurality of micro- and/or nano-scalefibers to form a second set of contacted tips;

Step 5: Picking up the second part and simultaneously maneuvering thefirst and second parts in position of a release location;

Step 6: Compressing the first and second sets of contacted tips of theplurality of micro- and/or nano-scale fibers until the first and secondsets of contacted tips lose contact with the first and the second partsand/or laterally shearing the first and second sets of contacted tips ofthe plurality of micro- and/or nano-scale fibers until the first andsecond sets of contacted tips lose contact with the first and secondparts; and

Step 7: Retracting the plurality of micro- and/or nano-scale fibers awayfrom the first and second parts to release the first and second partsfrom the manipulation device.

In operation, the present invention can manipulate parts from meterscale down to nanometer scale, planar, curved or any 3D shapedcomponents, biological specimens such as tissues, surgical films,organs, and cells, fragile components. The present invention canmanipulate a single part one at a time or many parts in parallel at atime as described above. The manipulation method can be used in transferprinting of flexible electronics components, semiconductor industrypick-and-place applications of chips, wafers, packaging components,biological specimen applications, car or other industry partpick-and-place applications, etc. The major advantages to the presentinvention are: 1) no damage to the parts; 2) no residue left on theparts; 3) manipulation can be made in vacuum, air, or liquid; 4) 3D partmanipulation is possible; 5) parallel manipulation is possible; 6) it iseasy to clean the fiber manipulator if contaminates; 7) the presentinvention method is simple and compact with minimal infrastructuralneeds with compared to suction and magnetic methods. The presentinvention manipulation method can be used as basic robotic positioningand orientation based discrete manipulation method or as a roll-to-rollbased continuous manufacturing method. The present invention fibers andmanipulation method can be used for enabling dry self-cleaning geckoadhesive structures since these fibers can pick dirt particles andrelease them on clean surfaces using the picking and releasing methodsdemonstrated here.

The present invention applies to fibers with any tip shape (for example,rounded, flat, symmetric or asymmetric flat mushroom, angled mushroom,etc.), any fiber angle (vertical or angled), any stem cross-sectionalgeometry (for example, cylindrical, triangular, pyramid, square,gradually getting smaller or larger, symmetric/asymmetric, tubularmicro/nanostructures such as nanotubes, nanowires, etc.), and anyfiber/tip material (for example, elastomers. polymers, any (carbon,polymer or metallic), nanotube materials, any nanowire materials, etc.).The core common property of the manipulation of the present invention isindependent from the fiber/tip geometry/material such that:

Pick: Have a good (fiber) tip contact area with the part to create goodadhesion to pick it up from the first substrate. Additionally, thepresent invention also used increased retraction speed during picking toincrease adhesion with the part.

Release: Reduce the tip contact area by loading the fiber verticallyand/or laterally to release the part easily on the second substrate.

Above methods apply to both a single fiber or a fiber array.

The patent application illustrates only cylindrical angled fibers withrounded and flat tips to pick and place parts as the experimentaldemonstrations while the manipulation method applies to all casesdiscussed above.

FIG. 11 illustrates examples of fiber tip forming operations:

Step a: Dip an array of angled fiber tips into a liquid polymer, whereineach angled fiber has a pre-set angle θ relative to a backing layer;

Step b: Remove the angled fiber tips from the liquid polymer;

Step c1: Apply a downward force onto the backing layer of the array tocontact the fiber tips with liquid polymer onto a substrate withoutcausing the array of angled fibers to bend or deflect beyond the pre-setangle θ;

Step d1: Remove the array of angled fiber tips from substrate after theliquid polymer cures to form symmetrical flat tips that are parallelwith the backing layer; or

Step c2: Apply a downward force onto the backing layer of the array tocontact the angled fiber tips with liquid polymer onto a substratecausing the angled fibers to bend or deflect by an angle 13 beyond itspre-set angle θ;

Step d2: Remove the fiber tips from substrate after the liquid polymercures to form symmetrical flat tips that are not parallel with thebacking layer at an angle β-θ.

To form asymmetric fiber tips, follow any of the preceding steps (a, b,c1 or a, b, c2), and then apply a lateral force onto the backing layerof the array and cure the liquid polymer to form asymmetric fiber tips(Step c3) and remove the fiber tips from the substrate after the liquidpolymer cures to form asymmetrical flat tips that are parallel with thebacking layer (Step d3). Though Step c3 in FIG. 11 is an embodimentassociated with Steps a, b, and c1, another embodiment can also beassociated with Steps a, c, and c2 as well to form asymmetrical flattips that are not parallel with the backing layer at an angle β-θ.

While the disclosure has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope of the embodiments. Thus, it isintended that the present disclosure cover the modifications andvariations of this disclosure provided they come within the scope of theappended claims and their equivalents.

We claim:
 1. A method for fabricating a single micro- and/or nano-scale fiber or plurality of micro- and/or nano-scale fibers with a flat tip comprising the steps of: providing a negative mold with a plurality of angled holes patterned in photoresist material disposed on a silicon substrate, wherein an outer surface of the silicon substrate is exposed between adjacent angled holes of the plurality of angled holes; pouring a liquid polymer into the negative mold to fill the plurality of angled holes, wherein the liquid polymer contacts the outer surface of the silicon substrate; curing the liquid polymer in the negative mold while in contact with the outer surface of the silicon substrate; and delaminating the cured polymer from the negative mold to form the micro- and/or nano-scale fiber with the flat tip.
 2. The method according to claim 1, further comprising the step of exposing the negative mold to dessicator vapor to facilitate delamination of the cure fiber material from the negative mold.
 3. The method according to claim 1, further comprising the step of disposing an anti-reflection layer on to the outer surface of the silicon substrate.
 4. The method according to claim 1, wherein the micro- and/or nano-scale fiber with the flat tip comprises a diameter from about 1 nm up to about 100 mm, a length from about 1 nm to about 100 mm, and a spacing between adjacent fibers from about 1 nm to 500 mm.
 5. The method according to claim 1, wherein the step of providing the negative mold comprising the steps of: mounting the silicon substrate with photoresist material on an angled state; exposing the photoresist material to ultraviolet (UV) light to pattern the plurality of angled holes; baking the photoresist material; and forming the plurality of angled holes in the photoresist material to produce the negative mold.
 6. The method according to claim 5, further comprising the step of spinning the photoresist materials onto the silicon substrate.
 7. The method according to claim 1, wherein the photoresist material is a layer having a thickness from about 10 nm up to about 10 mm.
 8. The method according to claim 5, further comprising the step of hard baking the negative mold.
 9. The method according to claim 8, wherein the step of hard baking the negative mold is performed at about 180° C. for 3 min to induce further crosslinking.
 10. A flat tip micro- and/or nano-scale fiber according to the method of claim 1 having a pick-to-release adhesive force ratio of about 1 to 1 up to about 100,000 to
 1. 11. A method for fabricating a single micro- and/or nano-scale fiber or a plurality of micro- and/or nano-scale fiber with a round tip comprising the steps of: providing a positive mold with a plurality of angled holes patterned in photoresist material disposed on a silicon substrate that form a plurality of angled round tip micro- and/or nanoscale pillars; pouring a first liquid polymer into the positive mold to fill the plurality of angled holes; curing the first liquid polymer in the positive mold to form a negative mold; delaminating the negative mold from the positive mold; pouring a second liquid polymer into the negative mold; curing the second liquid polymer in the negative mold; and delaminating the cured second liquid polymer from the negative mold to form the micro- and/or nano-scale fiber with the round tip.
 12. The method for fabricating a single micro- and/or nano-scale fiber or a plurality of micro- and/or nano-scale fiber with a round tip according to claim 11, further comprising a step of etching tips of a plurality of angled micro- and/or nanoscale pillars to form the plurality of angle round tip micro- and/or nanoscale pillars.
 13. The method for fabricating a micro- and/or nano-scale fiber with a round tip according to claim 11, wherein the liquid polymer is a thermoset elastomer.
 14. The method for fabricating a micro- and/or nano-scale fiber with a round tip according to claim 11, wherein the tip has a radius of from about 1 nm up to about 100 mm.
 15. The method for fabricating a micro- and/or nano-scale fiber with a round tip according to claim 11, wherein the micro- and/or nano-scale fiber with the round tip has a Young's modulus of from about 0.01 MPa to about 100,000 MPa and an effective work of adhesion on glass of from about 10 mJ m⁻² to about 2000 mJ m⁻².
 16. The method for fabricating a micro- and/or nano-scale fiber with a round tip according to claim 11, further comprising the step of molding the micro- and/or nano-scale fiber with the round tip on to a substrate to ease manual handling.
 17. The method for fabricating a micro- and/or nano-scale fiber with a round tip according to claim 16, wherein the substrate is selected from a group consisting of glass and acrylic to provide a rigid, transparent backing.
 18. The method for fabricating a micro- and/or nano-scale fiber with a round tip according to claim 16, further comprising the step of forming a backing layer disposed between the micro- and/or nano-scale fiber with the round tip and the substrate.
 19. The method for fabricating a micro- and/or nano-scale fiber with a round tip according to claim 18, wherein the backing layer has a thickness from about 1 nm up to about 100 mm between the micro- and/or nano-scale fiber with the round tip and the substrate.
 20. A round tip micro- and/or nano-scale fiber according to the method of claim 1 having a maximum pick-to-release adhesive force ratio of from about 1 to 1 up to about 100,000 to
 1. 21. A method of using micro- and/or nano-scale fibers to manipulate a first part relative to a second part, comprising the steps of: providing a manipulation device with a plurality of micro- and/or nano-scale fibers, wherein each micro- and/or nano-scale fiber has a tip; contacting the first part with one or more tips of the plurality of micro- and/or nano-scale fibers to form contacted tips; picking up the first part and maneuvering it in position of the second part; contacting the second part with the first part; reducing a contact area of each contacted tip of the contacted tips with the first part by compressing the contacted tips of the plurality of micro- and/or nano-scale fibers; and retracting the plurality of micro- and/or nano-scale fibers away from the first part to release the first part from the manipulation device.
 22. The method according to claim 21, wherein the step of contacting the first part with tips of the micro- and/or nano-scale fibers further comprises compressing the tips of the micro- and/or nano-scale fibers by from about 1 nm up to about 100 mm of compressive displacement.
 23. The method according to claim 21, further comprising the step of laterally shearing the tips during the step compressing the tips of the micro- and/or nano-scale fibers until the tips lose contact with the first part.
 24. The method according to claim 1, wherein the single micro- and/or nano-scale fiber with the flat tip has a Young's modulus of from about 0.01 MPa to about 100,000 MPa and an effective work of adhesion on glass of from about 10 mJ m⁻² to about 2000 mJ m⁻².
 25. A method of using micro- and/or nano-scale fibers to manipulate a first part relative to a second part, comprising the steps of: providing a manipulation device with a plurality of micro- and/or nano-scale fibers, wherein each micro- and/or nano-scale fiber has a tip; contacting the first part with one or more tips of the plurality of micro- and/or nano-scale fibers to form contacted tips; picking up the first part and maneuvering it in position of the second part; contacting the second part with the first part; reducing a contact area of each contacted tip of the contacted tips with the first part by laterally shearing the contacted tips of the plurality of micro- and/or nano-scale fibers; and retracting the plurality of micro- and/or nano-scale fibers away from the first part to release the first part from the manipulation device.
 26. The method according to claim 25, wherein the step of contacting the second part with tips of the micro- and/or nano-scale fibers further comprises compressing the tips of the micro- and/or nano-scale fibers by from about 1 nm up to about 100 mm of compressive displacement.
 27. The method according to claim 21, wherein the step of contacting the first part with one or more tips of the plurality of micro- and/or nano-scale fibers to form contacted tips and the step of compressing the contacted tips of the plurality of micro- and/or nano-scale fibers until the contacted tips lose contact with the first part further comprise the step of emitting monochromatic green light with a corresponding optical filter to visualize tip contact geometry and contact area of the tips of the plurality of micro- and/or nano-scale fibers.
 28. The method according to claim 25, wherein the step of contacting the first part with one or more tips of the plurality of micro- and/or nano-scale fibers to form contacted tips and the step of laterally shearing the contacted tips of the plurality of micro- and/or nano-scale fibers until the contacted tips lose contact with the first part further comprise the step of emitting monochromatic green light to visualize tip contact geometry and contact area of the tips of the plurality of micro- and/or nano-scale fibers.
 29. The method according to claim 21, further comprising the step of step of contacting a third part with one or more non-contacted tips of the micro- and/or nano-scale fibers.
 30. The method according to claim 25, further comprising the step of step of contacting a third part with one or more non-contacted tips of the micro- and/or nano-scale fibers.
 31. A method of using micro- and/or nano-scale fibers to manipulate a first part and a second part, comprising the steps of: providing a manipulation device with a plurality of micro- and/or nano-scale fibers, wherein each micro- and/or nano-scale fiber has a tip; contacting the first part with one or more tips of the plurality of micro- and/or nano-scale fibers to form a first set of contacted tips; picking up the first part and maneuvering it in position of the second part; contacting the second part with one or more non-contacted tips of the plurality of micro- and/or nano-scale fibers to form a second set of contacted tips; picking up the second part and simultaneously maneuvering the first and second parts in position of a release location; reducing a contact area of each contacted tip of the first and second sets of contacted tips with the first and second parts by compressing the first and second sets of contacted tips of the plurality of micro- and/or nano-scale fibers; and retracting the plurality of micro- and/or nano-scale fibers away from the first and second parts to release the first and second parts from the manipulation device.
 32. A method of using micro- and/or nano-scale fibers to manipulate a first part and a second part, comprising the steps of: providing a manipulation device with a plurality of micro- and/or nano-scale fibers, wherein each micro- and/or nano-scale fiber has a tip; contacting the first part with one or more tips of the plurality of micro- and/or nano-scale fibers to form a first set of contacted tips; picking up the first part and maneuvering it in position of the second part; contacting the second part with one or more non-contacted tips of the plurality of micro- and/or nano-scale fibers to form a second set of contacted tips; picking up the second part and simultaneously maneuvering the first and second parts in position of a release location; reducing a contact area of each contacted tip of the first and second sets of contacted tips with the first and second parts by laterally shearing the first and second sets of contacted tips of the plurality of micro- and/or nano-scale fibers; and retracting the plurality of micro- and/or nano-scale fibers away from the first and the second parts to release the first and second parts from the manipulation device.
 33. A method to form micro- and/or nano-scale fiber tips comprising the steps of: dipping an array of angled fiber tips into a liquid polymer, wherein each angled fiber has a pre-set angle θ relative to a backing layer; removing the angled fiber tips from the liquid polymer; applying a downward force onto a backing layer of the array to contact the array of angled fiber tips with liquid polymer onto a substrate without causing the angled fibers to bend or deflect beyond the pre-set angle θ; curing the liquid polymer; and removing the array of angled fiber tips from the substrate after the liquid polymer cures to form symmetrical flat tips that are parallel with the backing layer.
 34. A method to form micro- and/or nano-scale fiber tips comprising the steps of: dipping an array of angled fiber tips into a liquid polymer, wherein each angled fiber has a pre-set angle θ relative to a backing layer; removing the angled fiber tips from the liquid polymer; applying a downward force onto a backing layer of the array to contact the array of angled fiber tips with liquid polymer onto a substrate causing the angled fibers to bend or deflect by an angle β beyond the pre-set angle θ; curing the liquid polymer; and removing the array of angled fiber tips from the substrate after the liquid polymer cures to form symmetrical flat tips that are not parallel with the backing layer at an angle β-θ.
 35. A method to form micro- and/or nano-scale fiber tips comprising the steps of: dipping an array of angled fiber tips into a liquid polymer, wherein each angled fiber has a pre-set angle θ relative to a backing layer; removing the angled fiber tips from the liquid polymer; applying a downward force onto a backing layer of the array to contact the array of angled fiber tips with liquid polymer onto a substrate without causing the angled fibers to bend or deflect beyond the pre-set angle θ; applying a lateral force onto the backing layer of the array curing the liquid polymer; and removing the array of angled fiber tips from the substrate after the liquid polymer cures to form asymmetrical flat tips that are parallel with the backing layer.
 36. A method to form micro- and/or nano-scale fiber tips comprising the steps of: dipping an array of angled fiber tips into a liquid polymer, wherein each angled fiber has a pre-set angle θ relative to a backing layer; removing the angled fiber tips from the liquid polymer; applying a downward force onto a backing layer of the array to contact the array of angled fiber tips with liquid polymer onto a substrate causing the angled fibers to bend or deflect by an angle 13 beyond the pre-set angle θ; applying a lateral force onto the backing layer of the array curing the liquid polymer; and removing the array of angled fiber tips from the substrate after the liquid polymer cures to form asymmetrical flat tips that are not parallel with the backing layer at an angle β-θ.
 37. The method according to claim 21, wherein the step of picking up the first part and maneuvering it in position of the second part further comprises the step of retracting the plurality of micro- and/or nano-scale fibers at a retraction speed about 1 nanometer/second up to about 1 meter/second.
 38. The method according to claim 25, wherein the step of picking up the first part and maneuvering it in position of the second part further comprises the step of retracting the plurality of micro- and/or nano-scale fibers at a retraction speed about 1 nanometer/second up to about 1 meter/second.
 39. The method according to claim 31, wherein the step of picking up the first part and maneuvering it in position of the second part further comprises the step of retracting the plurality of micro- and/or nano-scale fibers at a retraction speed about 1 nanometer/second up to about 1 meter/second.
 40. The method according to claim 31, wherein the step of picking up the second part and simultaneously maneuvering the first and second parts in position of a release location further comprises the step of retracting the plurality of micro- and/or nano-scale fibers at a retraction speed about 1 nanometer/second up to about 1 meter/second.
 41. The method according to claim 32, wherein the step of picking up the first part and maneuvering it in position of the second part further comprises the step of retracting the plurality of micro- and/or nano-scale fibers at a retraction speed about 1 nanometer/second up to about 1 meter/second.
 42. The method according to claim 32, wherein the step of picking up the second part and simultaneously maneuvering the first and second parts in position of a release location further comprises the step of picking up the second part and simultaneously maneuvering the first and second parts in position of a release location. 